The present invention relates to a hydraulic steering assembly with change in transmission ratio and flow amplification, which includes a controllable control valve, a metering pump, a supply connection, a return connection and two cylinder connections.
Steering devices of this type are employed predominantly in slowly moving vehicles with high axle loads.
Hydraulic steering devices primarily include a rotary control valve and an orbit displacer. The rotary control valve is constructed of an outer control sleeve fitted in a housing and an inner control piston disposed concentrically in the control sleeve. The control piston is connected through an internal spline to the steering wheel and also through a pin connection to the control sleeve for relative movement thereto. The control sleeve is rigidly connected to the rotor gearwheel of the orbit displacer through a pin connection and a drive shaft. The control piston is centered relative to the control sleeve by way of a spring element. Control piston and control sleeve are able to rotate relative to each other within limits against the biasing force of this spring element. Commutator bore holes provide a hydraulic connection between the rotary control valve and the orbit displacer.
The hydraulic steering assembly includes connections for connecting to a supply line, to a return line and to two cylinder lines which are connected to the steering cylinder.
Although these hydraulic steering devices feature a high displacement volume, they are not capable of changing the transmission ratio between the power-assisted steering mode and the emergency steering mode. Consequently, if the supply pump fails and the steering assembly suddenly switches to the emergency mode, the entire steering force required to steer the vehicle has to be applied to the steering wheel manually. This is a safety risk, particularly in highway traffic.
For this reason, there is a long-felt need to provide steering units which make possible a change the transmission ratio when the emergency steering mode is engaged, thereby reducing the necessary manual steering force.